Process for the production of low carbon steel

ABSTRACT

A process for the production of low carbon steel from steel melts having a carbon content of about 0.04 percent and an oxygen content of between about 0.06 percent and 0.08 percent comprises vacuum degassing the melt in a vacuum chamber, preferably in portions, in the presence of granulated lime or limestone having a grain size of between 2 mm. and 1 cm., preferably between 2 mm. and 4 mm. and continuing the vacuum degassing treatment until a final carbon content in the melt of about 0.005 percent is reached.

United States Patent 72] Inventors Helmut Kni'ippel Sulzbach-Rosenberg Hutte; Karl Brotzmann, Sulzbach-Rosenberg, both of Germany [2]] Appl. No. 705,018 [22] Filed Feb. 13, 1968 [45] Patented Sept. 21, 1971 [73] Assignee Eisenwerk-Gesellschait Maximilianshutte m.b.H. [32] Priority Feb. 14, 1967 I 3 3 Germany [3 1 1233385 [54] PROCESS FOR THE PRODUCTION OF LOW CARBON STEEL 2 Claims, No Drawings [52] US. Cl 75/49, 75/93 [51] lnt.Cl. C2lc 7/10 [50] Field of Search 75/49 [56] References Cited UNITED STATES PATENTS 3,183,078 5/1965 Ohtake 75 49 3,230,074 1/1966 Roy etal 3,253,909 5/1966 Bishop et al. 75/49 X 3,375,100 3/1968 Vopel et a1. 75/49 3,417,463 12/1968 Knilppel et al 75/49 UX OTHER REFERENCES Technique of Organic Chemistry Vol. IV Distillation; Weissberger; lnterscience Publ. Co. N.Y.; 1951 pages 491- 493 Technique of Organic Chemistry Vol, IV Distillation; 2D Edition; 1965 page 515 Primary Examiner-L. Dewayne Rutledge Assistant Examiner.l. Davis Attorneys-Curt M. Avery, Arthur E. Wilfond, Herbert L,

Lerner and Daniel J. Tick PROCESS FOR THE PRODUCTION OF LOW CARBON STEEL The production on a large scale of steel melts containing approximately 0.005 percent carbon is becoming increasingly important industrially. Particularly great economic advantages are obtained when the product made from these steels takes the form of strip of the kind called electrical sheet, or strip for direct enamelling. In these low carbon steels the constancy of the carbon content is particularly important Attempts have been made to obtain these steels by applying a vacuum-metallurgical treatment to the molten steel. In this process the raw material is a steel melt produced in conventional refining apparatus and containing approximately 0.04 percent carbon and between 0.06 percent and 0.08 percent oxygen. In processing these melts the problem is to lower the carbon content to a low and reliably constant value for the subsequent manufacture of the most economically valuable products, for example electrical sheet or strip which will subsequently be given a coating of enamel. Hitherto the carbon in the degassed steels has fluctuated between 0.005 and 0.015 percent.

It has not hitherto been possible to o discover the reason for these fluctuations, although for theoretical reasons it has been supposed that the irregular carbon content is connected with the formation of nuclei for the gas bubbles. Attempts have been made to obtain an even degassing by introducing bubbles of gas into the melt in the vacuum vessel. These attempts have not been successful.

We have discovered that these fluctuations can be avoided, in accordance with the present invention, by vacuum degassing a steel melt containing substantially 0.04 percent carbon and between 0.06 percent and 0.08 percent oxygen in the presence of a refractory material granulated to a grain size of from substantially 2 mm. to substantially 0.005 percent.

Preferably the grain size of the granulated material is between 2 mm. and 4 mm. and the granulated refractory material is lime.

Some examples of processes in accordance with the invention will now be described in which the melt is degassed in portions by drawing one portion at a time into a vacuum chamber. This technique demonstrates the advantages of the process according to the invention particularly clearly, but the invention is not limited to this method of vacuum degassing steel.

In order to degas a 35-ton melt tapped from an LD crucible and containing 0.04 percent carbon and 0.070 percent oxygen, approximately l kg. of a screened burnt lime of a grain size between 2 and 4 mm. were introduced into a slag retainer of a vacuum vessel. The slag retainer is a conical structure mounted on the end of the pipe which extends from the bottom of the vacuum chamber, the slag retainer being mounted the end of the pipe which dips into the steel melt. For each portion of the melt treated approximately 6 tons of steel melt is introduced into the vacuum chamber. At the end of a total processing time of approximately 10 minutes, after the treatment of altogether 30 portions the final carbon content was brought down to 0.005 percent.

Under certain circumstances it can be advantageous to introduce either part of the lime or all the lime into the degassing chamber during the vacuum treatment. A particularly preferred method is to conduct the first part of the degassing treatment, during which large quantities of gas are released, this part of the process involving the treatment of between 10 and I5 portions in the example just described without adding any lime. The lime required to give the desired low final carbon content is then introduced into the degassing chamber either all at once or in portions, through the usual inlet air lock for additives.

The best operating program is determined from a pressure curve, showing pressure as a function of time. The effects of the addition of lime can be read 05 quite easily from the pressure increase due to the separation of gas.

A variation of this method has been found to be advantageous under certain circumstances. In this case the lime rs sprinkled on to the bottom of the degassing chamber before the beginning of the degassing treatment. However in this case the best effect is obtained by using a lime of larger grain size for example approximately 1 cm.

The above examples indicate the best operating conditions to bring the carbon content of a steel melt reliably down to substantially 0.005 percent by vacuum treatment. The best lime grain size is between 2 and 4mm. If a larger grain size is used, for example about 1 cm., a favorable effect is nevertheless obtained. For example, the operating conditions remaining the same as described above, an addition of 3 kg. of lime of grain size approximately 5 mm. considerably improves the removal of carbon from the steel.

The process according to the invention also minimizes the oxygen in melts of comparatively high carbon content, although this process is not so decisively important. Here again it is difficult to remove the last traces of gas.

It has furthermore been found that the lime additions have favorable effect on the working life of the refractory linings of the vacuum chamber. For example the working life of a magnesite lining in a l-leber vacuum chamber for degassing melts in portions has been increased in this way from melts to double this figure.

The process according to the invention is not limited to the use of granular lime. For example good results have been obtained by using granulated limestone and also granular clay.

We claim:

1. In a process for making steel having a low carbon content by vacuum degassing a steel melt containing about 0.04 percent carbon and between about 0.06 percent and about 0.08 percent oxygen, the improvement comprising introducing granulated lime having a grain size of from about 2 mm. to about 1 cm. into the bottom of a melt to be degassed and vacuum degassing until a final carbon content of about 0.005 percent is reached.

2. In a process for making steel having a low carbon content by vacuum degassing a steel melt containing about 0.04 percent carbon and between about 0.06 percent and about 0.08 percent oxygen, the improvement comprising introducing a granulated refractory material selected from lime, limestone and clay, and having a grain size of from about 2 mm. to about 4 mm. into the bottom of the melt and vacuum degassing until a final carbon content of about 0.005 percent is reached. 

2. In a process for making steel having a low carbon content by vacuum degassing a steel melt containing about 0.04 percent carbon and between about 0.06 percent and about 0.08 percent oxygen, the improvement comprising introducing a granulated refractory maTerial selected from lime, limestone and clay, and having a grain size of from about 2 mm. to about 4 mm. into the bottom of the melt and vacuum degassing until a final carbon content of about 0.005 percent is reached. 